Much of the prior art of matrix fixturing or potting centers on the use of low melting metallic alloys having controlled shrinkage characteristics. See, for example, U.S. Pat. Nos. 3,790,152 and 3,982,430. These materials provide a matrix for holding a wide range of metal parts for drilling, grinding, lapping, milling and turning operations. Certain metallic alloys in which bismuth or antimony are incorporated give molten metal alloys which expand upon solidification thus providing excellent duplication and reproduction. There are, however, many commercial fixturing operations where these metallic alloys do not provide optimum results. The weight of these alloys in fixturing large parts poses a processing problem; their high cost makes for an expensive inventory problem, and the loss of any of the alloy during processing is a serious economic disadvantage. Contamination caused by metal such as bismuth, cadmium, lead and tin contained in such materials is extremely damaging in the fixturing of jet engine parts, for example.
One of the major obstacles to wider utilization of thin wall, odd-shaped, fragile and nonmagnetic parts has been the relatively inflexible method by which they are mechanically potted for machining. Production difficulties in supporting these parts are caused by vibration and chatter, distortion, fracturing, deflection, interrupted cuts, rapid consumption of cutting tools, heavy burring, high scrap rate and secondary repairs. Typical material requirements for jet engine blade staging are as follows:
(1) The material should have the rigidity of lead at room temperature or below. PA1 (2) The material should be pourable and have a set up time of about 2-3 minutes. PA1 (3) The material should have minimum shrinkage and not be appreciably soluble in machining coolants. PA1 (4) Sequentially cast material should be mutually adherent. PA1 (5) The material should be easily removable from machined parts in such a manner as not to damage the parts. Any residual material should not attack the parts or coatings at temperatures as high as 2000.degree. F. PA1 (6) the material should be nontoxic, nonflammable and relatively odorless. PA1 (7) The material should be relatively inexpensive and reusable. PA1 (8) The material should have good shelf life and a pot life at application temperature.
In addition to the above, it is a principal objective in the formulation of the present improved potting compositions to provide relatively low viscosity at temperatures below 300.degree. F. preferably less than 260.degree. F. and a rapid cool down and hardening time.
Heretofore, organic compositions have been used in a potting process. For example, reference may be had to the U.S. Pat. No. to Lapac et al., 3,897,535. This patent discloses a process for potting a workpiece including disposing a liquid organic resin work holding material within a work holder around the workpiece and quenching the assembly at between 40.degree. F. and 73.degree. F. to solidify the work holding material. The cold quench within this particular temperature range gives improved holding strength and reduces shrinkage of the organic work holding material. According to the patented process, ceramic stones are heated to a relatively high temperature (800.degree. F.) and disposed within the work holder to heat the work holder and the workpiece prior to pouring the work holding material into the work holder. The stones further increase the holding strength of the work holding material and reduce shrinkage even more. Glass balls may be used in place of ceramic stone. These stones are of substantial diameter, preferably between one-eighth and two-tenths of an inch in diameter. Smaller sized stones or balls do not permit proper flow of wax into the work holding cavity. Stones or balls of a greater size while they may perform satisfactorily in heating the workpiece do not add much strength to the solidified composition. The organic resin material may be any wax or thermoplastic which is solid at room temperature.
Other references of interest in this field include Speyer U.S. Pat. Nos. 3,921,343; 3,748,155 and 3,854,962; Shepard 3,586,559 and McCormick 3,319,289.
The present invention is an improvement in an organic material for potting. By the process of the present invention and the composition utilized herein, the operations of separately filling the work holder with the ceramic stones or glass balls followed by pouring a molten fixturing composition thereover are avoided. The hazards of handling 800.degree. F. stones are avoided. Moreover, the necessity for recovering the glass balls or ceramic stones is obviated thereby reducing the number of operations and the cost of recovery of materials. Compositions of the present invention have all of the desired properties for potting of devices such as jet engine blades while achieving these objectives in a more economical and useful manner. The inorganic filler material does not need to be removed from the composition in order that the composition may be properly reused. The compositions hereof have excellent shelf life, and even when molten undergo minimum settling of the inorganic moiety. The improved elastomerized resin systems hereof provide hardness and toughness essential to tool handling during machining operations.
Still further, because of the relatively large size of the stones used in accordance with the practice of the prior art method and the relatively large sections of interstitial resin of low heat conductivity, the amount of time for quenching the workpiece container after being filled with the stones and the fixturing composition is still relatively high, e.g. 5-10 minutes. This unnecessarily extends the time for such operations as the fixturing and further working of jet engine blades, for example. In such an operation, time is of the essence and set-up time can be materially reduced, e.g. to as short as 2-3 minutes. The improved composition of the present invention and the improved potting process, enables the time of quenching to be substantially reduced over that heretofore required.